Could a cold heart stand a cold winter?

Could a cold heart stand a cold winter?

Recent dinosaur finds in southeastern Australia have scientists wondering how these ancient reptiles could have weathered the cold, dark winters that gripped that area 100 million years ago. While new evidence provides no solution to the freezing-dinosaur conundrum, it offers a potential way out of the problem.

According to the latest data, the winters may not have been as grueling as earlier evidence suggested, report Thomas H. Rich of the Museum of Victoria in Melbourne and Patricia V. Rich of Monash University in Clayton, Victoria. The paleontologists described their work this week at the 28th International Geological Congress, held in Washington, D.C.

The Riches and their colleagues have discovered a number of dinosaur fossils at sites along the coast of south-central Victoria (SN: 3/19/88, p.184). During mid-Cretaceous time, this region sat much closer to the South Pole and was attached to Antarctica.

To flesh out a picture of dinosaur lifestyle, paleontologists are reconstructing the former Victorian climate -- no easy feat for a time 100 million years ago. Living within the polar circle, the dinosaurs must have survived periods of winter darkness, which may have lasted up to four months, says Thomas Rich. Growth rings in tree fossils indicate the passing of seasons.

Using the ratios of oxygen isotopes in nearby rock, geochemists in the last few years have calculated the mean annual temperature of the fossil site at somewhere between -6[deg.]C and 5[deg.]C. Elsewhere in Australia, geologists have found evidence that ice existed at certain times of year during the Cretaceous (SN: 6/18/88, p.391). Since the Victoria dinosaurs were small and could not have migrated each season, the data hint these animals could have survived fairly cold, perhaps freezing temperatures--a suggestion that fuels the debate over whether dinosaurs were warm- or cold-blooded. Fossil finds from northern Alaska bolster the theory that dinosaurs could survive cold winters.

Yet the newest oxygen isotope data suggest that mean annual temperatures were 6[deg.]C, says Rich. What's more, his colleagues have just identified remains of a lizard from the same bone site. Lizards--known to be cold-blooded -- could not have survived freezing temperatures, he says.

While the evidence for warmer temperatures seems to contradict the earlier evidence, it may not. Just as North American temperatures have swung from cold to warm since the last ice age, the climate of Cretaceous Australia may have fluctuated over many millennia. If so, dinosaurs could have lived in warmer northern Australia during the cold period and moved south over thousands of years as the climate warmed. Alternatively, they may have remained in the south through the frigid times.

Researchers cannot yet determine whether dinosaurs survived cold winters in Australia, Rich says. The newest isotope samples come from rock layers nearer the bone beds, so they should give a better picture of the dinosaurs' habitat than did previous isotope data. However, even the closest samples are not contemporaneous with the bone beds. "They are separated by 2 to 3 meters vertically," he says. "That could still be a couple of millennia."